Normally, because it's a fan animation, I tend to be much more lenient in my comments. However, Zachary Rich helped produce this whole piece (along with 104 other people) with the intent to graduate from Savannah's College of Art and Design. This fan-made episode is out there to prove to everyone that he is ready to work in the professional field. Therefore, I'll review it as if it truly is a professionally published piece.

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This is a strong animated piece, with lots of action, characterization, and humor. Rich and crew took what is My Little Pony: Friendship Is Magic and created something that could fit within the realm of the canon while inserting of their own ideas. What are really strong is the following:

• Without question, the animation is the best part of Double Rainboom. One key component in animation is the fluidity of it, and it was extremely well-done. In some cases, it flowed a little better than some parts of the show and really pushed the boundaries of Adobe Flash, something DHX has been able to do.
  
• The characterization of Twilight Sparkle and Rainbow Dash are, for the most part, IN character, especially given the current timeline, which is somewhere in season two. Twilight retains that adorkable edge while freaking out, getting annoyed at the right moments, and caring at the right time. Rainbow Dash, meanwhile, retained that recklessness that she didn't dispel until Wonderbolts Academy. She likes to push herself to the edge, occasionally going too far, and used to fly very recklessly. In Read It and Weep, she flew so wildly, she crashed and broke her wing. Pinkie Pie's antics worked extremely well, retaining that cheery, bubbly confidence that everyone knows so well.
  
• The pop culture references are abundant, but don't intrude to the story. WeLoveFine, EQD, Toy Story, Harley Quinn, and so on blended right in the piece, and you have to really slow down in order to find it.
  
• The plot twist was shocking, but made a lot of sense. Whether you love it or hate it, NO one saw that crossover coming. But it was done in a way that works given the fantasy, folklore, and logic the canon provides. My Little Pony: Friendship Is Magic exploits their zany worldbuilding, but so does The Powerpuff Girls, and a lot of the crew worked on both projects. Lauren Faust, in part, worked as a storyboard artist in Powerpuff Girls, and the simplicity in PPG's characters and atmosphere are noticeable in FIM.
   
  At the same time, Rich and crew carefully researched various theories, especially the idea of exploring and ripping through an inter-dimensional, space-time continuum. There were bits and pieces foreshadowing the crossover, but there was one that, following the interview on Everfree Radio, gave it right away.
  • Twilight's contraption when holding Rainbow Dash back. I remembered seeing it once before when the preview was uploaded on YouTube a few weeks ago, but didn't know where. Rich explained that it came from the Powerpuff Girls episode, Bubblevicious (one of the most popular PPG episodes, by the way), and it rang in my head wondering how I missed that! It was subtle, but very clever.
    

  [*]There's another part of the professional world of MLP:FIM that shines: the background score. It shines here, too. Andrew Stein (MandoPony) did a splendid job with the music, giving it his flavor, but simultaneously blending in the canon. It's fast when needed, somber when needed, and hilarious when needed. It added so much flavor to Double Rainboom.

  [*]The humor is top-notch and follows the zany roots of both The Powerpuff Girls and My Little Pony: Friendship Is Magic. The funniest part is easily Rarity's hairdo becoming Twilight's, scaring away Spike.

  [*]There were plenty of brony references, but almost all of them came during specific moments where it didn't intrude to the plot. The Scootachicken joke belonged, and it was hilarious. Derpy had more secondary screentime than seasons one and three combined, desperately trying to eat her muffin (but couldn't); and you can't help but feel for her. WeLoveFine and EQD were there, but were so subtle and so quick that you couldn't spot them immediately. Berry Punch's brony reference for her alcoholism is there, too, but almost unnoticeable because it was in the background.

  [*]The tones for all of the characters match. The Powerpuff Girls were perfect. Twilight Sparkle, voiced by Rina-chan, closely associated with Strong's voice; and the same is said for Pinkie Pie. The only one that was half-and-half is Rainbow Dash. But despite the different voice, the way it was conveyed (her tone) matched her headstrong reckless character that was prevalent during seasons one and two.

  [*]The crew did a spectacular job portraying the consequences of the double rainboom. It was forceful and damaging. Ponyville was in shambles, as if in the middle of a nuclear explosion. They wanted it to be graphic, and they got it.

  [*]The end credits are akin to one of a Pixar film. Instead of being an afterthought, it's part of the episode, which is a very clever touch.

But there were also rather noticeable flaws.

• One brony reference was redundant: Snowflake's "YEAH!" Albeit funny, it ate up too much time on the clock.
  
• Some of the animation look like it was tweened too much. The biggest example is Fluttershy escorting the chickens before Rainbow Dash zoomed by — It was rather choppy.
  
• Blossom, Bubbles, and Buttercup's lines were way too simple to display much genuine character. The only moment where the character was displayed was Buttercup's aggression during the fight with the monster.
  
• The script was to the point, but it was actually too simple. While many of the lines were in character, some didn't have the depth necessary to make the characterization as convincing as it should, especially the Powerpuff Girls'. Better lines from them and more hilarious conversation between the PPGs and Dash (and maybe Professor Utonium, if you want to brainstorm) would provide such great chemistry.
  
• The explanation for the double rainboom was nonexistant. Where did it come from? It's evident through the script's effects that, if going fast enough to break through the speed of sound AND light, a double rainboom causes a rift in the continuum, landing the individual who's going through it into another dimension. But there was never any history or backstory behind it in the form of the actual script. It came off as extremely abrupt, and we still never got the full details behind them. In other words, given the current writing, it's a glaring plothole.
  
• Twilight's mere anger following Rainbow Dash's return didn't match. She was worried that Dash would kill herself, and then couldn't find her. She showed absolutely NO relief that Rainbow Dash is back in one piece, just frustration and anger, and it didn't fit her one bit. A combination of both (combined with disappointment) would've been much more in character.
  
• The deus ex machina, while fitting in the canon and Pinkie's "fourth wall breaking" was stupid! Despite the foreshadowing early in Double Rainboom, there was absolutely NO buildup for it, and it made no sense, even in the context of the letter.
  
• The letter to Celestia fit: Dash drank the potion without thinking ahead and resulted in some serious consequences. But the letter felt very incomplete and didn't have the right enough polish for it to make it very convincing. (The reference with Pinkie entering through the "FL" door was hilarious, though.)
  
• The episode is too short and didn't have the balance between Rainbow Dash's hilarious scurrying in Townsville and Twilight's method to get her back to Ponyville. It was exclusively Rainbow Dash's side of the story, and part of the plot suffered from it. Double Rainboom's plot would've been much better if the perspective was more omniscient here, focusing on both Rainbow Dash and Twilight. If the perspective was balanced, then Pinkie Pie's portal traveling wouldn't be such a rough deus ex machina, and Pinkie Pie's inclusion would've made more logical sense.
  

In short, Double Rainboom had very great polish in its animation, and there was a lot of heart. There were minute, but very glaring issues in the executions of the PPG's characterization and concepts that bogged down Double Rainboom, so what could've been even greater wasn't. Nevertheless, for a long production, Rich and crew did a marvelous job laying down the groundwork needed for the fandom to come up with future material. He shows that he is ready to go out into the professional world of animation once he graduates, and I can't wait to see what other ideas he has in mind in the future. Very good work!

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Source: Double Rainboom